Optical screen



Oct. 9, 1962 R. "r. ERBAN 3,057,256

OPTICAL SCREEN Original Filed March 10, 1952 IN V EN TOR.

United States Patent 9 3,057,256 OPTICAL SCREEN Richard T. Erban, 144-11Sanford Ave, Flushing, N.Y. Original application Mar. 10, 1952, Ser. No.275,760, now Patent No. 2,780,136, dated Feb. 5, 1957. Divided and thisapplication June 8, 1956, Ser. No. 590,121 3 Claims. (Cl. 88-4833) Thepresent invention relates to optical screens and more particularly to ascreen which will reflect incident light substantially back in the samedirection from which it arrives for 'an appreciable range of angles ofincidence.

This application is a division of my copending application, Serial No.275,760 filed on March 10, 1952, now Patent No. 2,780,136 issued onFebruary 5, 1957. Application Serial No. 275,760, in turn, is acontinuation-inpart of application No. 778,571 filed on October 8, 1947and now Patent No. 2,588,373 issued on March 11, 1952.

More particularly, this invention relates to projection screens and thelike. Where it is desired to have a screen that will show an imageprojected onto it from the side opposite to the side seen by theobserver (so called rear projection screen) considerable diflicultiesarise as a result of outside light which reaches the observers side ofthe screen and is reflected from there into the eyes of the observer.

This outside or stray light raises the general level of illumination ofthe screen, or its apparent brightness all over the screen, so that darkand bright areas have their level lifted by the same amount; the netresult is a decrease in contrast between light and dark areas, which, ifit goes too far, will practically obliterate fine details from theimage. T o overcome this, screens of this type have heretofore been usedmostly in rooms with strongly reduced light level, or covered by shadowboxes. Also various means have been proposed in order to reduce thisreflected light by reducing the reflectivity of the screen surface byanti-reflection coatings, or by special filters positioned between thescreen surface and the observer.

The present invention on the contrary provides a means in which thereflectivity of the screen surface is wholly immaterial as long as it isof the specular type, that is one in which little or no scattering ofthe reflected light occurs. Its screen surface is composed of amultitude of elements which are so constructed that all outside lightthat impinges upon the screen is reflected back in such directions whichwill avoid the eyes of the observer. This is accomplished by giving theelements the form of a hollow tri-cornered pyramid, such as illustratedin FIG. 1 of the accompanying drawing. The peculiarity of this pyramidconsists in the fact that the three surfaces are inclined with respectto each other at exactly 90.

FIGURE 1 shows a hollow tri-cornered pyramid greatly enlarged.

FIGURE 3 shows a plurality of tri-cornered pyramids formed from theintersection of three grooves.

FIGURE 2 shows a cross section through one of the grooves.

It can be shown that any ray of light, which impinges upon such anelement from anywhere within its apex will successively strike all 3surfaces and then be sent back parallel to the direction in which itcame. If the hollow pyramid is very small, the light ray will bereturned exactly to the point from which it came. This excludespractically the pupils of the eyes of the observer since they do notradiate light.

A system of a multitude of these tri-cornered pyramids, nested into eachother without any dead space between them can be obtained by theintersection of 3 systems of parallel, triangular grooves of a specificangle of inclination between their sides. The 3 systems of grooves mustintersect each other at angles of exactly 120, as illustrated to thisinvention, the groove angle alpha is given by the equation:

cotan /2 alpha equal to square root of two When grooves with this valueof groove angle are brought to intersection at 120, which may be done bycutting rows or such grooves into a flat plate, then the material leftstanding will form rows of tri-cornered pyramids, as shown, and the sideangles of the surfaces will be for each of the individual pyramids.These pyramids will be positives that is, not hollow pyramids; a metalplate so cut is then to be used as a die upon suitable plastic material,and it will leave an imprint of a multitude of little hollow pyramids,joining each other on their upper edges of the triangular opening, asmay be seen from FIG. 3.

It is to be noted that the relative size of the grooves and pyramids inall figures have been exaggerated for the purpose of clarity ofdisclosure and that for practical applica tion the grooves may be madevery much smaller, as explained in my copending application.

While I have described a specific form of embodiment of my invention, itis understood that this is done by way of example and that modificationmay be brought to my invention without departing from its basicstructure; and all such modification shall be understood to come withinthe scope of my invention which shall be limited only as defined by thefollowing claims:

What I claim is:

1. In an optical projection device, a translucent rear projection screenadapted to be positioned with one of its surfaces facing an observer andthe other of its surfaces being positioned to receive a projected image,the screen surface which faces the observer being composed entirely ofcontiguous equal triple pyramid-shaped recesses having 90 dihedralangles between adjacent faces of each individual recess, whereby lightreaching said one surface from any extraneous source is retroflectivelydirected back in the direction of its arrival and stray-light effects byambient light on said screen are at least partially avoided.

2. In an optical projection system for viewing projected images underconditions of ambient light, a translucent rear projection screen havinga surface adapted to make visible a projected optical real image to theeyes of an observer, and means adapted to prevent ambient light frombeing reflected into the eyes of said observer, said means comprising asurface facing the observer and which is composed entirely of contiguousrecesses, each recess being in the form of a triple pyramid having 90dihedral angles between each of its three faces.

3. In an optical projection system for viewing projected images underconditions of disturbing ambient light, a translucent rear projectionscreen having a surface adapted to make visible to the eyes of anobserver the projected optical real image, and means for reducing theeffects of ambient stray-light on said screen, said means beingpositioned in proximity to said screen and comprising a specularlyretroflective transparent surface facing the observer and which iscomposed entirely of contiguous triple pyramid-shaped recesses each ofwhich has 90 dihedral angles between each of its adjacent faces.

(References on following page) fief'e'ren'c'es' Cited in the file ofthis patent UNITED STATES PATENTS Stimson July 6, 1926 cawley Dec. 14,1-926 Buchner 1- Apr, 17, 1928 0 Martinek et al Mar. 13, 1934 4 Bull eta1 Aug. 14, 1934 Allison June 13, 1939 Jungersen July 31, 1945 WilsonJan. 16, 1951 Erban Mar. 11, 1952 Gear Dec. 16, 1952

